Dipole Moments of the Lowest Singlet π* ← π States in Phenol and Aniline by the Optical Stark Effect

The Stark effect in the rotational fine structure is used to measure the change in dipole moment on excitation in aniline and phenol. The spectra are in the 0–0 bands of the lowest‐lying singlet $\mathrm{\pi *\hspace{0.17em}\leftarrow \hspace{0.17em}\pi }$ states. The results are $\text{|\hspace{0.17em}Δμ\hspace{0.17em}|\hspace{0.17em}=\hspace{0.17em}0.85\hspace{0.17em}±\hspace{0.17em}0.15}\mathrm{D}$ for aniline and $\text{|\hspace{0.17em}Δμ\hspace{0.17em}|\hspace{0.17em}=\hspace{0.17em}0.20\hspace{0.17em}±\hspace{0.17em}0.20}\mathrm{D}$ for phenol. Evidence is presented which indicates that the sign of $\mathrm{\Delta \mu }$ is positive. If this is true, these results correlate well with the structural changes on excitation, namely, that the contraction along the in‐plane long axis is considerably greater in aniline than phenol. This correlation is strong indication of the extent of contribution of quinoidlike resonance structures to the electronic nature of the excited state.